There are requirements in fiber optic systems for precise control of optical signal levels entering various system components. This is particularly true for systems at test and characterization stages of deployment. A controllable optical attenuator can be used, for example, to characterize and optimize the optoelectronic response of high-speed photoreceivers, wherein the detection responsivity is dependent on the average optical power incident on the photodiode.
The majority of controllable fiber optic attenuators currently commercially available rely on thin-film absorption filters, which require breaking the fiber and placing the filters in-line. Controllable attenuation is then achieved by mechanical means such as rotating or sliding the filter to change the optical path length within the absorptive material. This adversely impacts the response speed of the device, the overall mechanical stability, zero attenuation insertion loss and optical back reflection. In general, broken fiber designs suffer numerous disadvantages such as high insertion loss, significant back reflection, and large size. These factors can be minimized, although such corrective measures typically result in added cost and/or size.
Additional issues have impeded the development of thermo-optic variable attenuators, including: (i) the thermal mass of surrounding materials and/or structures which significantly degrades device response time; and (ii) spectrally non-uniform attenuation, resulting from a dispersion mis-match between the optical mode index of the underlying transmission media and a controllable overlay material.
Improved controllable fiber optic attenuators and attenuation systems are therefore required which keep the optical fiber core intact, which achieve controllable attenuation via control of radiative loss from the fiber, and which offer improved response time and spectral uniformity over the wavelength bands of interest.